The inferior colliculus occupies a pivotal position in the central auditory system; it receives direct inputs from all of the major auditory brainstem nuclei and, in turn, it provides nearly all of the input to the auditory forebrain. Anatomical evidence suggests that the brainstem projections to the central nucleus of the inferior colliculus (ICC) form highly organized synaptic domains with both segregated and shared sources of input. The long-term objective of this research is to determine how these convergence patterns create discrete pathways for the optimal processing of biologically relevant acoustic information. In support of this parallel processing model, our recent electrophysiological studies have discovered three principal ICC response types that appear to be uniquely specialized for the neural encoding of spectral cues for sound localization, narrowband signals in noise, and binaural level and timing information. Based on correlations with response properties in lower-order nuclei, it has been hypothesized that each ICC unit type reflects a dominant excitatory input from either the medial superior olive, the lateral superior olive or the dorsal cochlear nucleus. Experiments of Aims 1-3 in this application are designed to provide direct evidence for theses functional connections. Our proposed multidisciplinary approach combines single-unit recording techniques, cross-correlation analyses, and pharmacological manipulations to determine the relative strength and topographic distribution of the putative inputs to the different ICC response types. After these initial investigations describe the anatomical pathways that link the auditory midbrain to the brainstem, experiments of Aim 4 will explore the functional consequences of this synaptic organization by comparing the quantity of acoustic representations in ICC target neurons and their sources of input. A question of particular interest in these later experiments is how the ascening inputs to the ICC interact with each other and a rich intrinsic inhibitory circuitry to enhance the processing of sound localization information. This research will provide currently unknown details regarding auditory processing within the inferior colliculus, and will establish a basic theoretical foundation for future electrophysiological studies of information streaming within the central nervous system.